How to remove webbing from curved mesh?
Actually you've filled the bottom and top edges with a simple F (fill), but as the shape is round it goes bad because the center of the whole face is out of this semi-circular shape. So you should keep quads:


Select those faces and press X (delete) > Faces.
Now select all these vertices except the extreme right and left ones.
Press W > Bridge Edge Loops. It will fill the gap between the edges.
Then finish the process, select the extreme right vertices and press F. Same for the extreme left vertices.
Do the exact same process for the opposite edge.